Copyright © 2003 -- 2006 by Juliusz Chroboczek.

Polipo
******

Polipo is a caching web proxy designed to be used as a personal cache or a cache shared among a few users.

This manual was written by Juliusz Chroboczek.

Background
**********

The web and HTTP
================

The web is a wide-scale decentralised distributed hypertext system, something that's obviously impossible to achieve reliably.

The web is a collection of resources which are identified by URLs, strings starting with http://.  At any point in time, a resource has a certain value, which is called an instance of the resource.

The fundamental protocol of the web is HTTP, a simple request/response protocol.  With HTTP, a client can make a request for a resource to a server, and the server replies with an entity, which is an on-the-wire representation of an instance or of a fragment thereof.

Proxies and caches
==================

A proxy is a program that acts as both a client and a server.  It listens for client requests and forwards them to servers, and forwards the servers' replies to clients.

An HTTP proxy can optimise web traffic away by caching server replies, storing them in memory in case they are needed again.  If a reply has been cached, a later client request may, under some conditions, be satisfied without going to the source again.

In addition to taking the shortcuts made possible by caching, proxies can improve performance by generating better network traffic than the client applications would do.

Proxies are also useful in ways unrelated to raw performance.  A proxy can be used to contact a server that are not visible to the browser, for example because there is a firewall in the way (see Parent proxies), or because the client and the server use different lower layer protocols (for example IPv4 and IPv6).  Another common application of proxies is to modify the data sent to servers and returned to clients, for example by censoring headers that expose too much about the client's identity (see Censoring headers) or removing advertisements from the data returned by the server (see Forbidden).

Polipo is a caching HTTP proxy that was originally designed as a personal proxy, i.e. a proxy that is used by a single user or a small group of users.

Latency and throughput
======================

Most network benchmarks consider throughput, or the average amount of data being pushed around per unit of time.  While important for batch applications (for example benchmarks), average throughput is mostly irrelevant when it comes to interactive web usage.  What is more important is a transaction's median latency, or whether the data starts to trickle down before the user gets annoyed.

Typical web caches optimise for throughput --- for example, by consulting sibling caches before accessing a remote resource.  By doing so, they significantly add to the median latency, and therefore to the average user frustration.

Polipo was designed to minimise latency.

Network traffic
===============

The web was developed by people who were interested in text processing rather than in networking and, unsurprisingly enough, the first versions of the HTTP protocol did not make very good use of network resources.  The main problem in HTTP/0.9 and early versions of HTTP/1.0 was that a separate TCP connection (``virtual circuit'' for them telecom people) was created for every entity transferred.

Opening multiple TCP connections has significant performance implications.  Obviously, connection setup and teardown require additional packet exchanges which increase network usage and, more importantly, latency.

Less obviously, TCP is not optimised for that sort of usage.  TCP aims to avoid network congestion, a situation in which the network becomes unusable due to overly aggressive traffic patterns.  A correct TCP implementation will very carefully probe the network at the beginning of every connection, which means that a TCP connection is very slow during the first couple of kilobytes transferred, and only gets up to speed later.  Because most HTTP entities are small (in the 1 to 10 kilobytes range), HTTP/0.9 uses TCP where it is most inefficient.

Persistent connections
----------------------

Later HTTP versions allow the transfer of multiple entities on a single connection.  A connection that carries multiple entities is said to be persistent (or sometimes keep-alive). Unfortunately, persistent connections are an optional feature of HTTP, even in version 1.1.

Polipo will attempt to use persistent connections on the server side, and will honour persistent connection requests from clients.

Pipelining
----------

With persistent connections it becomes possible to pipeline or stream requests, i.e. to send multiple requests on a single connection without waiting for the replies to come back.  Because this technique gets the requests to the server faster, it reduces latency. Additionally, because multiple requests can often be sent in a single packet, pipelining reduces network traffic.

Pipelining is a fairly common technique (The X11 protocol fundamentally relies on pipelining.  NNTP does support pipelining. SMTP doesn't, while ESMTP makes it an option.  FTP does support pipelining on the control connection.), but it is not supported by HTTP/1.0.  HTTP/1.1 makes pipelining support compulsory in every server implementation that can use persistent connections, but there are a number of buggy servers that claim to implement HTTP/1.1 but don't support pipelining.

Polipo carefully probes for pipelining support in a server and uses pipelining if it believes that it is reliable.  Polipo also deeply enjoys being pipelined at by a client (Other client-side implementations of HTTP that make use of pipelining include Opera, recent versions of Mozilla, APT (the package downloader used by Debian GNU/Linux) and LFTP.).

Poor Man's Multiplexing
-----------------------

A major weakness of the HTTP protocol is its inability to share a single connection between multiple simultaneous transactions --- to multiplex a number of transactions over a single connection.  In HTTP, a client can either request all instances sequentially, which significantly increases latency, or else open multiple concurrent connections, with all the problems that this implies (see Persistent connections).

Poor Man's Multiplexing (PMM) is a technique that simulates multiplexing by requesting an instance in multiple segments; because the segments are fetched in independent transactions, they can be interleaved with requests for other resources.

Obviously, PMM only makes sense in the presence of persistent connections; additionally, it is only effective in the presence of pipelining (see Pipelining).

PMM poses a number of reliability issues.  If the resource being fetched is dynamic, it is quite possible that it will change between segments; thus, an implementation making use of PMM needs to be able to switch to full-resource retrieval when it detects a dynamic resource.

Polipo supports PMM, but it is disabled it by default (see PMM).

Caching partial instances
=========================

A partial instance is an instance that is being cached but only part of which is available in the local cache.  There are three ways in which partial instances can arise: client applications requesting only part of an instance (Adobe's Acrobat Reader plugin is famous for that), a server dropping a connection mid-transfer (because it is short on resources, or, surprisingly often, because it is buggy), a client dropping a connection (usually because the user pressed stop).

When an instance is requested that is only partially cached, it is possible to request just the missing data by using a feature of HTTP known as a range request.  While support for range requests is optional, most servers honour them in case of static data (data that are stored on disk, rather then being generated on the fly e.g. by a CGI script).

Caching partial instances has a number of positive effects.  Obviously, it reduces the amount of data transmitted as the available data needn't be fetched again.  Because it prevents partial data from being discarded, it makes it reasonable for a proxy to unconditionally abort a download when requested by the user, and therefore reduces network traffic.

Polipo caches arbitrary partial instances in its in-memory cache.  It will only store the initial segment of a partial instance (from its beginning up to its first hole) in its on-disk cache, though.  In either case, it will attempt to use range requests to fetch the missing data.

Other requests
==============

The previous sections pretend that there is only one kind of request in HTTP --- the GET request.  In fact, there are some others.

The HEAD request method retrieves data about an resource.  Polipo does not normally use HEAD, but will fall back to using it for validation it if finds that a given server fails to cooperate with its standard validation methods (see Cache transparency).  Polipo will correctly reply to a client's HEAD request.

The POST method is used to request that the server should do something rather than merely sending an entity; it is usually used with HTML forms that have an effect (HTML forms should use the GET method when the form has no side-effect as this makes the results cacheable.).  The PUT method is used to replace an resource with a different instance; it is typically used by web publishing applications.

POST and PUT requests are handled by Polipo pretty much like GET and HEAD; however, for various reasons, some precautions must be taken.  In particular, any cached data for the resource they refer to must be discarded, and they can never be pipelined.

Finally, HTTP/1.1 includes a convenient backdoor with the CONNECT method.  For more information, please see Tunnelling connections.

Polipo does not currently handle the more exotic methods such as OPTIONS and PROPFIND.

Other HTTP proxies
==================

I started writing Polipo because the weather was bad.  But also because I wanted to implement some features that other web proxies don't have.

Harvest and Squid
-----------------

Harvest, the grandfather of all web caches, has since evolved into Squid.

Squid sports an elegant single-threaded non-blocking architecture and multiplexes multiple clients in a single process.  It also features almost complete support for HTTP/1.1, although for some reason it doesn't currently advertise it.

Squid is designed as a large-scale shared proxy running on a dedicated machine, and therefore carries certain design decisions which make it difficult to use as a personal proxy.  Because Squid keeps all resource meta-data in memory, it requires a fair amount of RAM in order to manipulate a reasonably sized cache.

Squid doesn't cache partial instances, and has trouble with instances larger than available memory (Recent versions of Squid support instances larger than available memory by using a hack that the authors call a ``sliding window algorithm''.).  If a client connection is interrupted, Squid has to decide whether to continue fetching the resource (and possibly waste bandwidth) or discard what it already has (and possibly waste bandwidth).

Some versions of squid would, under some circumstances, pipeline up to two outgoing requests on a single connection.  At the time of writing, this feature appears to have been disabled in the latest version.

Squid's developers have decided to re-write it in C++.

The Apache proxy
----------------

The Apache web server includes a complete HTTP/1.1 proxy.

The Apache web server was designed to maximise ease of programming --- a decision which makes Apache immensely popular for deploying web-based applications.  Of course, this ease of programming comes at a cost, and Apache is not the most lightweight proxy available.

As cheaper caching proxies are available, Apache is not useful as a standalone proxy.  The main application of Apache's proxy is to join multiple web servers' trees into a single hierarchy.

The Apache proxy doesn't cache partial instances and doesn't pipeline multiple outgoing requests.

WWWOFFLE
--------

WWWOFFLE, an elegant personal proxy, is the primary model for Polipo.

WWWOFFLE has more features than can be described here.  It will censor banner ads, clean your HTML, decorate it with random colours, schedule fetches for off-peak hours.

Unfortunately, the HTTP traffic that WWWOFFLE generates is disgusting. It will open a connection for every fetch, and forces the client to do the same.

WWWOFFLE only caches complete instances.

I used WWWOFFLE for many years, and frustration with WWWOFFLE's limitations is the main reason why I started Polipo in the first place.

Junkbuster
----------

Junkbuster is a simple non-caching web proxy designed to remove banner ads and cookies.  It was the main model for WWWOFFLE's (and therefore Polipo's) header and ad-removing features.

Junkbuster's HTTP support is very simple (some would say broken): it doesn't do persistent connections, and it breaks horribly if the client tries pipelining.  Junkbuster is no longer being maintained, and has evolved into Privoxy.

Privoxy
-------

Privoxy is the current incarnation of Junkbuster.  Privoxy has the ability to randomly modify web pages before sending them to the browser --- for example, remove <blink> or <img> tags.

Just like its parent, Privoxy cannot do persistent connections.  Under some circumstances, it will also buffer whole pages before sending them to the client, which significantly adds to its latency.  However, this is difficult to avoid given the kinds of rewriting it attempts to perform.

Oops
----

Oops is a caching web proxy that uses one thread (lightweight process) for every connection. This technique does cost additional memory, but allows good concurrency of requests while avoiding the need for complex non-blocking programming.  Oops was apparently designed as a wide-scale shared proxy.

Although Oops' programming model makes it easy to implement persistent connections, Oops insists on opening a separate connection to the server for every single resource fetch, which disqualifies it from production usage.

Running Polipo
**************

Starting Polipo
===============

By default, Polipo runs as a normal foreground job in a terminal in which it can log random ``How do you do?'' messages.  With the right configuration options, Polipo can run as a daemon.

Polipo is run with the following command line:

     $ polipo [ -h ] [ -v ] [ -x ] [ -c CONFIG ] [ VAR=VAL... ]

All flags are optional.  The flag -h causes Polipo to print a short help message and to quit.  The flag -v causes Polipo to list all of its configuration variables and quit.  The flag -x causes Polipo to purge its on-disk cache and then quit (see Purging).  The flag -c specifies the configuration file to use (by default ~/.polipo or /etc/polipo/config).  Finally, Polipo's configuration can be changed on the command line by assigning values to given configuration variables.

Configuration
-------------

There is a number of variables that you can tweak in order to configure Polipo, and they should all be described in this manual (see Variable index).  You can display the complete, most up-to-date list of configuration variables by using the -v command line flag or by accessing the ``current configuration'' page of Polipo's web interface (see Web interface).  Configuration variables can be set either on the command line or else in the configuration file given by the -c command-line flag.

Configuration variables are typed, and -v will display their types.  The type can be of one of the following:

  * integer or float: a numeric value;

  * boolean: a truth value, one of true or false;

  * tristate: one of false, maybe or true;

  * 4-state, one of false, reluctantly, happily or true;

  * 5-state, one of false, reluctantly, maybe, happily or true;

  * atom, a string written within double quotes ");

  * list, a comma-separated list of strings;

  * intlist, a comma-separated list of integers and ranges of integers (of the form `N--M').

The configuration file has a very simple syntax.  All blank lines are ignored, as are lines starting with a hash sign #.  Other lines must be of the form

     VAR = VAL

where VAR is a variable to set and VAL is the value to set it to.

It is possible to change the configuration of a running polipo by using the local configuration interface (see Web interface).

Running as a daemon
-------------------

If the configuration variable daemonise is set to true, Polipo will run as a daemon: it will fork and detach from its controlling terminal (if any).  The variable daemonise defaults to false.

When Polipo is run as a daemon, it can be useful to get it to atomically write its pid to a file.  If the variable pidFile is defined, it should be the name of a file where Polipo will write its pid.  If the file already exists when it is started, Polipo will refuse to run.

Logging
-------

When it encounters a difficulty, Polipo will print a friendly message. The location where these messages go is controlled by the configuration variables logFile and logSyslog. If logSyslog is true, error messages go to the system log facility given by logFacility.  If logFile is set, it is the name of a file where all output will accumulate.  If logSyslog is false and logFile is empty, messages go to the error output of the process (normally the terminal).

The variable logFile defaults to empty if daemonise is false, and to /var/log/polipo otherwise.

The variable logSyslog defaults to false, and logFacility defaults to user.

The amount of logging is controlled by the variable logLevel. Please see the file log.h in the Polipo sources for the possible values of logLevel.

(Note that if you are using a shared cache, or logging to syslog, an overly low logLevel could be a privacy violation.)

Configuring your browser
========================

Telling your user-agent (web browser) to use Polipo is an operation that depends on the browser.  Many user-agents will transparently use Polipo if the environment variable http_proxy points at it; e.g.

     $ export http_proxy=http://localhost:8123/

Netscape Navigator, Mozilla, Mozilla Firefox, KDE's Konqueror and probably other browsers require that you configure them manually through their Preferences or Configure menu.

If your user-agent sports such options, tell it to use persistent connections when speaking to proxies, to speak HTTP/1.1 and to use HTTP/1.1 pipelining.

Stopping Polipo and getting it to reload
========================================

Polipo will shut down cleanly if it receives SIGHUP, SIGTERM or SIGINT signals; this will normally happen when a Polipo in the foreground receives a ^C key press, when your system shuts down, or when you use the kill command with no flags.  Polipo will then write-out all its in-memory data to disk and quit.

If Polipo receives the SIGUSR1 signal, it will write out all the in-memory data to disk (but won't discard them), reopen the log file, and then reload the forbidden URLs file (see Forbidden).

Finally, if Polipo receives the SIGUSR2 signal, it will write out all the in-memory data to disk and discard as much of the memory cache as possible.  It will then reopen the log file and reload the forbidden URLs file.

The local web server
====================

Polipo includes a local web server, which is accessible on the same port as the one the proxy listens to.  Therefore, by default you can access Polipo's local web server as http://localhost:8123/.

The data for the local web server can be configured by setting localDocumentRoot, which defaults to /usr/share/polipo/www/.  Setting this variable to "" will disable the local server.

Polipo assumes that the local web tree doesn't change behind its back. If you change any of the local files, you will need to notify Polipo by sending it a SIGUSR2 signal (see Stopping).

If you use polipo as a publicly accessible web server, you might want to set the variable disableProxy, which will prevent it from acting as a web proxy.  (You will also want to set disableLocalInterface (see Web interface), and perhaps run Polipo in a chroot jail.)

The web interface
-----------------

The subtree of the local web space rooted at http://localhost:8123/polipo/ is treated specially: URLs under this root do not correspond to on-disk files, but are generated by Polipo on-the-fly.  We call this subtree Polipo's local web interface.

The page http://localhost:8123/polipo/config? contains the values of all configuration variables, and allows setting most of them.

The page http://localhost:8123/polipo/status? provides a summary status report about the running Polipo, and allows performing a number of actions on the proxy, notably flushing the in-memory cache.

The page http://localhost:8123/polipo/servers? contains the list of known servers, and the statistics maintained about them (see Server statistics).

The pages starting with http://localhost:8123/polipo/index? contain indices of the disk cache.  For example, the following page contains the index of the cached pages from the server of some random company:

     http://localhost:8123/polipo/index?http://www.microsoft.com/

The pages starting with http://localhost:8123/polipo/recursive-index? contain recursive indices of various servers.  This functionality is disabled by default, and can be enabled by setting the variable disableIndexing.

If you have multiple users, you will probably want to disable the local interface by setting the variable disableLocalInterface. You may also selectively control setting of variables, indexing and listing known servers by setting the variables disableConfiguration, disableIndexing and disableServersList.

Polipo and the network
**********************

Client connections
==================

There are three fundamental values that control how Polipo speaks to clients.  The variable proxyAddress, defines the IP address on which Polipo will listen; by default, its value is the loopback address "127.0.0.1", meaning that Polipo will listen on the IPv4 loopback interface (the local host) only.  By setting this variable to a global IP address or to one of the special values "::" or "0.0.0.0", it is possible to allow Polipo to serve remote clients.  This is likely to be a security hole unless you set allowedClients to a reasonable value (see Access control).

Note that the type of address that you specify for proxyAddress will determine whether Polipo listens to IPv4 or IPv6.  Currently, the only way to have Polipo listen to both protocols is to specify the IPv6 unspecified address ("::") for proxyAddress.

The variable proxyPort, by default 8123, defines the TCP port on which Polipo will listen.

The variable proxyName, which defaults to the host name of the machine on which Polipo is running, defines the name of the proxy.  This can be an arbitrary string that should be unique among all instances of Polipo that you are running.  Polipo uses it in error messages and optionally for detecting proxy loops (by using the Via HTTP header, see Censoring headers).

Access control
--------------

By making it possible to have Polipo listen on a non-routable address (for example the loopback address 127.0.0.1), the variable proxyAddress provides a very crude form of access control: the ability to decide which hosts are allowed to connect.

A finer form of access control can be implemented by specifying explicitly a number of client addresses or ranges of addresses (networks) that a client is allowed to connect from.  This is done by setting the variable allowedClients.

Every entry in allowedClients can be an IP address, for example 134.157.168.57 or ::1.  It can also be a network address, i.e. an IP address and the number of bits in the network prefix, for example 134.157.168.0/24 or 2001:660:116::/48.  Typical uses of allowedClients variable include

     allowedClients = 127.0.0.1, ::1, 134.157.168.0/24, 2001:660:116::/48

or, for an IPv4-only version of Polipo,

     allowedClients = 127.0.0.1, 134.157.168.0/24

A different form of access control can be implemented by requiring each client to authenticate, i.e. to prove its identity before connecting.  Polipo currently only implements the most insecure form of authentication, HTTP basic authentication, which sends usernames and passwords in clear over the network.  HTTP basic authentication is required when the variable authCredentials is not null; its value should be of the form username:password.

Note that both IP-based authentication and HTTP basic authentication are insecure: the former is vulnerable to IP address spoofing, the latter to replay attacks.  If you need to access Polipo over the public Internet, the only secure option is to have it listen over the loopback interface only and use an ssh tunnel (see Parent proxies) (It is not quite clear to me whether HTTP digest authentication is worth implementing.  On the one hand, if implemented correctly, it appears to provide secure authentication; on the other hand, and unlike ssh or SSL, it doesn't make any attempt at ensuring privacy, and its optional integrity guarantees are impossible to implement without significantly impairing latency.).

Contacting servers
==================

A server can have multiple addresses, for example if it is multihomed (connected to multiple networks) or if it can speak both IPv4 and IPv6.  Polipo will try all of a hosts addresses in turn; once it has found one that works, it will stick to that address until it fails again.

If connecting via IPv6 there is the possibility to use temporary source addresses to increase privacy (RFC 3041). The variable useTemporarySourceAddress controls the use of temporary addresses for outgoing connections; if set to true temporary addresses are preferred, if set to false static addresses are used and if set to maybe (the default) the operation system default is in effect. This setting is not available on all operation systems.

Allowed ports
-------------

A TCP service is identified not only by the IP address of the machine it is running on, but also by a small integer, the TCP port it is listening on.  Normally, web servers listen on port 80, but it is not uncommon to have them listen on different ports; Polipo's internal web server, for example, listens on port 8123 by default.

The variable allowedPorts contains the list of ports that Polipo will accept to connect to on behalf of clients; it defaults to 80-100, 1024-65535.  Set this variable to 1-65535 if your clients (and the web pages they consult!) are fully trusted.  (The variable allowedPorts is not considered for tunnelled connections; see Tunnelling connections).

Tuning at the HTTP level
========================

Tuning the HTTP parser
----------------------

As a number of HTTP servers and CGI scripts serve incorrect HTTP headers, Polipo uses a lax parser, meaning that incorrect HTTP headers will be ignored (a warning will be logged by default).  If the variable laxHttpParser is not set (it is set by default), Polipo will use a strict parser, and refuse to serve an instance unless it could parse all the headers.

When the amount of headers exceeds one chunk's worth (see Chunk memory), Polipo will allocate a big buffer in order to store the headers.  The size of big buffers, and therefore the maximum amount of headers Polipo can parse, is specified by the variable bigBufferSize (32kB by default).

Censoring headers
-----------------

Polipo offers the option to censor given HTTP headers in both client requests and server replies.  The main application of this feature is to very slightly improve the user's privacy by eliminating cookies and some content-negotiation headers.

It is important to understand that these features merely make it slightly more difficult to gather statistics about the user's behaviour.  While they do not actually prevent such statistics from being collected, they might make it less cost-effective to do so.

The general mechanism is controlled by the variable censoredHeaders, the value of which is a case-insensitive list of headers to unconditionally censor.  By default, it is empty, but I recommend that you set it to From, Accept-Language.  Adding headers such as Set-Cookie, Set-Cookie2, Cookie, Cookie2 or User-Agent to this list will probably break many web sites.

The case of the Referer (HTTP contains many mistakes and even one spelling error.) header is treated specially because many sites will refuse to serve pages when it is not provided.  If censorReferer is false (the default), Referer headers are passed unchanged to the server.  If censorReferer is maybe, Referer headers are passed to the server only when they refer to the same host as the resource being fetched.  If censorReferer is true, all Referer headers are censored.  I recommend setting censorReferer to maybe.

Another header that can have privacy implications is the Via header, which is used to specify the chain of proxies through which a given request has passed.  Polipo will generate Via headers if the variable disableVia is false (it is true by default).  If you choose to generate Via headers, you may want to set the proxyName variable to some innocuous string (see Client connections).

Why censor Accept-Language
..........................

Recent versions of HTTP include a mechanism known as content negotiation which allows a user-agent and a server to negotiate the best representation (instance) for a given resource.  For example, a server that provides both PNG and GIF versions of an image will serve the PNG version to user-agents that support PNG, and the GIF version to Internet Explorer.

Content negotiation requires that a client should send with every single request a number of headers specifying the user's cultural and technical preferences.  Most of these headers do not expose sensitive information (who cares whether your browser supports PNG?).  The Accept-Language header, however, is meant to convey the user's linguistic preferences.  In some cases, this information is sufficient to pinpoint with great precision the user's origins and even his political or religious opinions; think, for example, of the implications of sending Accept-Language: yi or ar_PS.

At any rate, Accept-Language is not useful.  Its design is based on the assumption that language is merely another representation for the same information, and Accept-Language simply carries a prioritised list of languages, which is not enough to usefully describe a literate user's preferences.  A typical French user, for example, will prefer an English-language original to a French (mis-)translation, while still wanting to see French language texts when they are original.  Such a situation cannot be described by the simple-minded Accept-Language header.

Offline browsing
================

In an ideal world, all machines would have perfect connectivity to the network at all times and servers would never crash.  In the real world, it may be necessary to avoid hitting the network and have Polipo serve stale objects from its cache.

Setting proxyOffline to true prevents Polipo from contacting remote servers, no matter what.  This setting is suitable when you have no network connection whatsoever.

If proxyOffline is false, Polipo's caching behaviour is controlled by a number of variables documented in Tweaking validation.

Server statistics
=================

In order to decide when to pipeline requests (see Pipelining) and whether to perform Poor Man's Multiplexing (see Poor Mans Multiplexing), Polipo needs to keep statistics about servers.  These include the server's ability to handle persistent connections, the server's ability to handle pipelined requests, the round-trip time to the server, and the server's transfer rate.  The statistics are accessible from Polipo's web interface (see Web interface).

The variable serverExpireTime (default 1 day) specifies how long such information remains valid.  If a server has not been accessed for a time interval of at least serverExpireTime, information about it will be discarded.

As Polipo will eventually recover from incorrect information about a server, this value can be made fairly large.  The reason why it exists at all is to limit the amount of memory used up by information about servers.

Tweaking server-side behaviour
==============================

The most important piece of information about a server is whether it supports persistent connections.  If this is the case, Polipo will open at most serverSlots connections to that server (serverSlots1 if the server only implements HTTP/1.0), and attempt to pipeline; if not, Polipo will hit the server harder, opening up to serverMaxSlots connections.

Another use of server information is to decide whether to pipeline additional requests on a connection that already has in-flight requests.  This is controlled by the variable pipelineAdditionalRequests; if it is false, no additional requests will be pipelined.  If it is true, additional requests will be pipelined whenever possible.  If it is maybe (the default), additional requests will only be pipelined following small requests, where a small request one whose download is estimated to take no more than smallRequestTime (default 5s).

Sometimes, a request has been pipelined after a request that prompts a very large reply from the server; when that happens, the pipeline needs be broken in order to reduce latency.  A reply is large and will cause a pipeline to be broken if either its size is at least replyUnpipelineSize (default one megabyte) or else the server's transfer rate is known and the body is expected to take at least replyUnpipelineTime to download (default 15s).

The variable maxPipelineTrain defines the maximum number of requests that will be pipelined in a single write (default 10). Setting this variable to a very low value might (or might not) fix interaction with some unreliable servers that the normal heuristics are unable to detect.

The variable maxSideBuffering specifies how much data will be buffered in a PUT or POST request; it defaults to 1500 bytes.  Setting this variable to 0 may cause some media players that abuse the HTTP protocol to work.

Poor Man's Multiplexing
=======================

By default, Polipo does not use Poor Man's Multiplexing (see Poor Mans Multiplexing).  If the variable pmmSize is set to a positive value, Polipo will use PMM when speaking to servers that are known to support pipelining.  It will request resources by segments of pmmSize bytes.  The first segment requested has a size of pmmFirstSize, which defaults to twice pmmSize.

PMM is an intrinsically unreliable technique.  Polipo makes heroic efforts to make it at least usable, requesting that the server disable PMM when not useful (by using the If-Range header) and disabling it on its own if a resource turns out to be dynamic. Notwithstanding these precautions, unless the server cooperates (More precisely, unless CGI scripts cooperate.), you will see failures when using PMM, which will usually result in blank pages and broken image icons; hitting Reload on your browser will usually cause Polipo to notice that something went wrong and correct the problem.

Forbidden and redirected URLs
=============================

The web contains advertisements that a user-agent is supposed to download together with the requested pages.  Not only do advertisements pollute the user's brain, pushing them around takes time and uses up network bandwidth.

Many so-called content providers also track user activities by using web bugs, tiny embedded images that cause a server to log where they are requested from.  Such images can be detected because they are usually uncacheable (see Cache transparency) and therefore logged by Polipo by default.

Polipo can be configured to prevent certain URLs from reaching the browser, either by returning a forbidden error message to the user, or by redirecting such URLs to some other URL.

Internal forbidden list
-----------------------

The file pointed at by the variable forbiddenFile (defaults to ~/.polipo-forbidden or /etc/polipo/forbidden, whichever exists) specifies the set of URLs that should never be fetched.  If forbiddenFile is a directory, it will be recursively searched for files with forbidden URLs.

Every line in a file listing forbidden URLs can either be a domain name --- a string that doesn't contain any of /, * or \ ---, or a POSIX extended regular expression.  Blank lines are ignored, as are those that start with a hash sign #.

By default, whenever it attempts to fetch a forbidden URL, the browser will receive a 403 forbidden error from Polipo.  Some users prefer to have the browser display a different page or an image.

If forbiddenUrl is not null, it should represent a URL to which all forbidden URLs will be redirected.  The kind of redirection used is specified by forbiddenRedirectCode; if this is 302 (the default) the redirection will be marked as temporary, if 301 it will be a permanent one.

External redirectors
--------------------

Polipo can also use an external process (a Squid-style redirector) to determine which URLs should be redirected.  The name of the redirector binary is determined from the variable redirector, and the kind of redirection generated is specified by redirectorRedirectCode, which should be 302 (the default) or 301.

For example, to use Adzapper to redirect ads to an innocuous image, just set

     redirector = /usr/bin/adzapper

The domain name service
=======================

The low-level protocols beneath HTTP identify machines by IP addresses, sequences of four 8-bit integers such as 199.232.41.10 (Or sequences of eight 16-bit integers if you are running IPv6.).  HTTP, on the other hand, and most application protocols, manipulate host names, strings such as www.polipo.org.

The domain name service (DNS) is a distributed database that maps host names to IP addresses.  When an application wants to make use of the DNS, it invokes a resolver, a local library or process that contacts remote name servers.

Polipo usually tries to speak the DNS protocol itself rather than using the system resolver (The Unix interface to the resolver is provided by the gethostbyname(3) library call (getaddrinfo(3) on recent systems), which was designed at a time when a host lookup consisted in searching for one of five hosts in a HOSTS.TXT file.  The gethostbyname call is blocking, meaning that all activity must cease while a host lookup is in progress.  When the call eventually returns, it doesn't provide a time to live (TTL) value to indicate how long the address may be cached.  For these reasons, gethostbyname is hardly useful for programs that need to contact more than a few hosts. (Recent systems replace gethostbyname(3) by getaddrinfo(3), which is reentrant.  While this removes one important problem that multi-threaded programs encounter, it doesn't solve any of the other issues with gethostbyname.)).  Its precise behaviour is controlled by the value of dnsUseGethostbyname.  If dnsUseGethostbyname is false, Polipo never uses the system resolver.  If it is reluctantly (the default), Polipo tries to speak DNS and falls back to the system resolver if a name server could not be contacted. If it is happily, Polipo tries to speak DNS, and falls back to the system resolver if the host couldn't be found for any reason (this is not a good idea for shared proxies).  Finally, if dnsUseGethostbyname is true, Polipo never tries to speak DNS itself and uses the system resolver straight away (this is not recommended).

If the internal DNS support is used, Polipo must be given a recursive name server to speak to.  By default, this information is taken from the /etc/resolv.conf file; however, if you wish to use a different name server, you may set the variable dnsNameServer to an IP address (While Polipo does its own caching of DNS data, I recommend that you run a local caching name server.  I am very happy with pdnsd, notwithstanding its somewhat bizarre handling of TCP connections.).

When the reply to a DNS request is late to come, Polipo will retry multiple times using an exponentially increasing timeout.  The maximum timeout used before Polipo gives up is defined by dnsMaxTimeout (default 60s); the total time before Polipo gives up on a DNS query will be roughly twice dnsMaxTimeout.

The variable dnsNegativeTtl specifies the time during which negative DNS information (information that a host doesn't exist) will be cached; this defaults to 120s.  Increasing this value reduces both latency and network traffic but may cause a failed host not to be noticed when it comes back up.

The variable dnsQueryIPv6 specifies whether to query for IPv4 or IPv6 addresses.  If dnsQueryIPv6 is false, only IPv4 addresses are queried.  If dnsQueryIPv6 is reluctantly, both types of addresses are queried, but IPv4 addresses are preferred. If dnsQueryIPv6 is happily (the default), IPv6 addresses are preferred.  Finally, if dnsQueryIPv6 is true, only IPv6 addresses are queried.

If the system resolver is used, the value dnsGethostbynameTtl specifies the time during which a gethostbyname reply will be cached (default 5 minutes).

Parent proxies
==============

Polipo will usually fetch instances directly from source servers as this configuration minimises latency.  In some cases, however, it may be useful to have Polipo fetch instances from a parent proxy.

Polipo can use two protocols to speak to a parent proxy: HTTP and SOCKS.  When configured to use both HTTP and SOCKS proxying, Polipo will contact an HTTP proxy over SOCKS --- in other words, SOCKS is considered as being at a lower (sub)layer than HTTP.

HTTP parent proxies
-------------------

The variable parentProxy specifies the hostname and port number of an HTTP parent proxy; it should have the form host:port.

If the parent proxy requires authorisation, the username and password should be specified in the variable parentAuthCredentials in the form username:password.  Only Basic authentication is supported, which is vulnerable to replay attacks.

The main application of the parent proxy support is to cross firewalls.  Given a machine, say trurl, with unrestricted access to the web, the following evades a firewall by using an encrypted compressed ssh link:

     $ ssh -f -C -L 8124:localhost:8123 trurl polipo
     $ polipo parentProxy=localhost:8124

SOCKS parent proxies
--------------------

The variable socksParentProxy specifies the hostname and port number of a SOCKS parent proxy; it should have the form host:port.  The variant of the SOCKS protocol being used is defined by socksProxyType, which can be either socks4a or socks5; the latter value specifies ``SOCKS5 with hostnames'', and is the default.

The user name passed to the SOCKS4a proxy is defined by the variable socksUserName.  This value is currently ignored with a SOCKS5 proxy.

The main application of the SOCKS support is to use Tor to evade overly restrictive or misconfigured firewalls.  Assuming you have a Tor client running on the local host listening on the default port (9050), the following uses Tor for all outgoing HTTP traffic:

     $ polipo socksParentProxy=localhost:9050

Tuning POST and PUT requests
============================

The main assumption behind the design of the HTTP protocol is that requests are idempotent: since a request can be repeated by a client, a server is allowed to drop a connection at any time.  This fact, more than anything else, explains the amazing scalability of the protocol.

This assumption breaks down in the case of POST requests.  Indeed, a POST request usually causes some action to be performed (a page to be printed, a significant amount of money to be transferred from your bank account, or, in Florida, a vote to be registered), and such a request should not be repeated.

The only solution to this problem is to reserve HTTP to idempotent activities, and use reliable protocols for action-effecting ones. Notwithstanding that, HTTP/1.1 makes a weak attempt at making POST requests slightly more reliable and efficient than they are in HTTP/1.0.

When speaking to an HTTP/1.1 server, an HTTP client is allowed to request that the server check a priori whether it intends to honour a POST request.  This is done by sending an expectation, a specific header with the request, Expect: 100-continue, and waiting for either an error message or a 100 Continue reply from the server.  If the latter arrives, the client is welcome to send the rest of the POST request (This, of course, is only part of the story.  Additionally, the server is not required to reply with 100 Continue, hence the client must implement a timeout. Furthermore, according to the obsolete RFC2068, the server is allowed to spontaneously send 100 Continue, so the client must be prepared to ignore such a reply at any time.).

Polipo's behaviour w.r.t. client expectations is controlled by the variable expectContinue.  If this variable is false, Polipo will never send an expectation to the server; if a client sends an expectation, Polipo will fail the expectation straight away, causing the client (if correctly implemented) to retry with no expectation. If expectContinue is maybe (the default), Polipo will behave in a standards-compliant manner: it will forward expectations to the server when allowed to do so, and fail client expectations otherwise.  Finally, if expectContinue is true, Polipo will always send expectations when it is reasonable to do so; this violates the relevant standards and will break some websites, but might decrease network traffic under some circumstances.

Tunnelling connections
======================

Polipo is an HTTP proxy; it proxies HTTP traffic, and clients using other protocols should either establish a direct connection to the server or use an ad hoc proxy.

In many circumstances, however, it is not possible to establish a direct connection to the server, for example due to mis-configured firewalls or when trying to access the IPv4 Internet from an IPv6-only host.  In such situations, it is possible to have Polipo behave as a tunnelling proxy --- a proxy that merely forwards traffic between the client and the server without understanding it.  Polipo enters tunnel mode when the client requests it by using the HTTP CONNECT method.

Most web browsers will use this technique for HTTP over SSL if configured to use Polipo as their `https proxy'.  More generally, the author has successfully used it to cross mis-configured firewalls using OpenSSH, rsync, Jabber, IRC, etc.

The variable tunnelAllowedPorts specifies the set of ports that Polipo will accept to tunnel traffic to.  It defaults to allowing ssh, HTTP, https, rsync, IMAP, imaps, POP, pops, Jabber, CVS and Git traffic.

Caching
*******

Cache transparency and validation
=================================

If resources on a server change, it is possible for a cached instance to become out-of date.  Ideally, a cache would be perfectly transparent, meaning that it never serves an out-of-date instance; in a universe with a finite speed of signal propagation, however, this ideal is impossible to achieve.

If a caching proxy decides that a cached instance is new enough to likely still be valid, it will directly serve the instance to the client; we then say that the cache decided that the instance is fresh.  When an instance is stale (not fresh), the cache will check with the upstream server whether the resource has changed; we say that the cached instance is being revalidated.

In HTTP/1.1, responsibility for revalidation is shared between the client, the server and the proxy itself.  The client can override revalidation policy by using the Cache-Control header (Or the obsolete Pragma header.); for example, some user-agents will request end-to-end revalidation in this way when the user shift-clicks on reload.  The server may choose to specify revalidation policy by using the Expires and Cache-Control headers.  As to the proxy, it needs to choose a revalidation policy for instances with neither server- nor client-side cache control information.  Of course, nothing (except the HTTP/1.1 spec, but that is easily ignored) prevents a proxy from overriding the client's and server's cache control directives.

Tuning validation behaviour
---------------------------

Polipo's revalidation behaviour is controlled by a number of variables.  In the following, an resource's age is the time since it was last validated, either because it was fetched from the server or because it was revalidated.

The policy defining when cached instances become stale in the absence of server-provided information is controlled by the variables maxAge, maxAgeFraction, maxExpiresAge and maxNoModifiedAge.  If an instance has an Expires header, it becomes stale at the date given by that header, or when its age becomes larger than maxExpiresAge, whichever happens first.  If an instance has no Expires header but has a LastModified header, it becomes stale when its age reaches either maxAgeFraction of the time since it was last modified or else the absolute value maxAge, whichever happens first.  Finally, if an instance has neither Expires nor Last-Modified, it will become stale when its age reaches maxNoModifiedAge.

Further tweaking of validation behaviour
----------------------------------------

If cacheIsShared is false (it is true by default), Polipo will ignore the server-side Cache-Control directives private, s-maxage and proxy-must-revalidate.  This is highly desirable behaviour when the proxy is used by just one user, but might break some sites if the proxy is shared.

When connectivity is very poor, the variable relaxTransparency can be used to cause Polipo to serve stale instances under some circumstances.  If relaxTransparency is false (the default), all stale instances are validated (see Cache transparency), and failures to connect are reported to the client. This is the default mode of operation of most other proxies, and the least likely to surprise the user.

If relaxTransparency is maybe, all stale instances are still validated, but a failure to connect is only reported as an error if no data is available in the cache.  If a connection fails and stale data is available, it is served to the client with a suitable HTTP/1.1 Warning header.  Current user-agents do not provide visible indication of such warnings, however, and this setting will typically cause the browser to display stale data with no indication that anything went wrong.  It is useful when you are consulting a live web site but don't want to be bothered with failed revalidations.

If relaxTransparency is true, missing data is fetched from remote servers, but stale data are unconditionally served with no validation.  Client-side Cache-Control directives are still honoured, which means that you can force an end-to-end revalidation from the browser's interface (typically by shift-clicking on ``reload'').  This setting is only useful if you have very bad network connectivity or are consulting a very slow web site or one that provides incorrect cache control information (This is for example the case of www.microsoft.com, and also of websites generated by a popular Free content management system written in Python.) and are willing to manually revalidate pages that you suspect are stale.

If mindlesslyCacheVary is true, the presence of a Vary header (which indicates that content-negotiation occurred, see Censor Accept-Language) is ignored, and cached negotiated instances are mindlessly returned to the client.  If it is false (the default), negotiated instances are revalidated on every client request.

Unfortunately, a number of servers (most notably some versions of Apache's mod_deflate module) send objects with a ETag header that will confuse Polipo in the presence of a Vary header.  Polipo will make a reasonable check for consistency if dontTrustVaryETag is set to maybe (the default); it will systematically ignore ETag headers on objects with Vary headers if it is set to true.

A number of websites incorrectly mark variable resources as cachable; such issues can be worked around in polipo by manually marking given categories of objects as uncachable.  If dontCacheCookies is true, all pages carrying HTTP cookies will be treated as uncachable. If dontCacheRedirects is true, all redirects (301 and 302) will be treated as uncachable.  Finally, if everything else fails, a list of uncachable URLs can be given in the file specified by uncachableFile, which has the same format as the forbiddenFile (see Internal forbidden list).  If not specified, its location defaults to ~/.polipo-uncachable or /etc/polipo/uncachable, whichever exists.

The in-memory cache
===================

The in-memory cache consists of a list of HTTP and DNS objects maintained in least-recently used order.  An index to the in-memory cache is maintained as a (closed) hash table.

When the in-memory cache grows beyond a certain size (controlled by a number of variables, see Memory usage), or when a hash table collision occurs, resources are written out to disk.

The on-disk cache
=================

The on-disk cache consists in a filesystem subtree rooted at a location defined by the variable diskCacheRoot, by default "/var/cache/polipo/".  This directory should normally be writeable, readable and seekable by the user running Polipo.  While it is best to use a local filesystem for the on-disk cache, a NFSv3- or AFS-mounted filesystem should be safe in most implementations.  Do not use NFSv2, as it will cause cache corruption  (Polipo assumes that open(O_CREAT | O_EXCL) works reliably.).

If diskCacheRoot is an empty string, no disk cache is used.

The value maxDiskEntries (32 by default) is the absolute maximum of file descriptors held open for on-disk objects.  When this limit is reached, Polipo will close descriptors on a least-recently-used basis.  This value should be set to be slightly larger than the number of resources that you expect to be live at a single time; defining the right notion of liveness is left as an exercise for the interested reader.

The value diskCacheWriteoutOnClose (32kB by default) is the amount of data that Polipo will write out when closing a disk file.  Writing out data when closing a file can avoid subsequently reopening it, but causes unnecessary work if the instance is later superseded.

The integers diskCacheDirectoryPermissions and diskCacheFilePermissions are the Unix filesystem permissions with which files and directories are created in the on-disk cache; they default to 0700 and 0600 respectively.

The variable maxDiskCacheEntrySize specifies the maximum size, in bytes, of an instance that is stored in the on-disk cache.  If set to -1 (the default), all objects are stored in the on-disk cache,

Asynchronous writing
--------------------

When Polipo runs out of memory (see Limiting memory usage), it will start discarding instances from its memory cache.  If a disk cache has been configured, it will write out any instance that it discards.  Any memory allocation that prompted the purge must then wait for the write to complete.

In order to avoid the latency hit that this causes, Polipo will preemptively write out instances to the disk cache whenever it is idle.  The integer idleTime specifies the time during which Polipo will remain idle before it starts writing out random objects to the on-disk cache; this value defaults to 20s.  You may want to decrease this value for a busy cache with little memory, or increase it if your cache is often idle and has a lot of memory.

The value maxObjectsWhenIdle (default 32) specifies the maximum number of instances that an idle Polipo will write out without checking whether there's any new work to do.  The value maxWriteoutWhenIdle specifies the maximum amount of data (default 64kB) that Polipo will write out without checking for new activity.  Increasing these values will make asynchronous write-out slightly faster, at the cost of possibly increasing Polipo's latency in some rare circumstances.

Purging the on-disk cache
-------------------------

Polipo never removes a file in its on-disk cache, except when it finds that the instance that it represents has been superseded by a newer version.  In order to keep the on-disk cache from growing without bound, it is necessary to purge it once in a while.  Purging the cache typically consists in removing some files, truncating large files (see Partial instances) or moving them to off-line storage.

Polipo itself can be used to purge its on-disk cache; this is done by invoking Polipo with the -x flag.  This can safely be done when Polipo is running (see Modifying the on-disk cache).

For a purge to be effective, it is necessary to cause Polipo to write-out its in-memory cache to disk (see Stopping). Additionally, Polipo will not necessarily notice the changed files until it attempts to access them; thus, you will want it to discard its in-memory cache after performing the purge.  The safe way to perform a purge is therefore:

     $ kill -USR1 POLIPO-PID
     $ sleep 1
     $ polipo -x
     $ kill -USR2 POLIPO-PID

The behaviour of the -x flag is controlled by three configuration variables.  The variable diskCacheUnlinkTime specifies the time during which an on-disk entry should remain unused before it is eligible for removal; it defaults to 32 days.

The variable diskCacheTruncateTime specifies the time for which an on-disk entry should remain unused before it is eligible for truncation; it defaults to 4 days and a half.  The variable diskCacheTruncateSize specifies the size at which files are truncated after they have not been accessed for diskCacheTruncateTime; it defaults to 1MB.

Usually, Polipo uses a file's modification time in order to determine whether it is old enough to be expirable.  This heuristic can be disabled by setting the variable preciseExpiry to true.

Format of the on-disk cache
---------------------------

The on-disk cache consists of a collection of files, one per instance. The format of an on-disk resource is similar to that of an HTTP message: it starts with an HTTP status line, followed by HTTP headers, followed by a blank line (\r\n\r\n).  The blank line is optionally followed by a number of binary zeroes.  The body of the instance follows.

The headers of an on-disk file have a few minor differences with HTTP messages.  Obviously, there is never a Transfer-Encoding line. A few additional headers are used by Polipo for its internal bookkeeping:

  * X-Polipo-Location: this is the URL of the resource stored in this file.  This is always present.

  * X-Polipo-Date: this is Polipo's estimation of the date at which this instance was last validated, and is used for generating the Age header of HTTP messages.  This is optional, and only stored if different from the instance's date.

  * X-Polipo-Access: this is the date when the instance was last accessed by Polipo, and is used for cache purging (see Purging). This is optional, and is absent if the instance was never accessed.

  * X-Polipo-Body-Offset: the presence of this line indicates that the blank line following the headers is followed by a number of zero bytes.  Its value is an integer, which indicates the offset since the beginning of the file at which the instance body actually starts. This line is optional, and if absent the body starts immediately after the blank line.

Modifying the on-disk cache
---------------------------

It is safe to modify the on-disk cache while Polipo is running as long as no file is ever modified in place.  More precisely, the only safe operations are to unlink (remove, delete) files in the disk cache, or to atomically add new files to the cache (by performing an exclusive open, or by using one of the link or rename system calls).  It is not safe to truncate a file in place.

Memory usage
************

Polipo uses two distinct pools of memory, the chunk pool and the malloc pool.

Chunk memory
============

Most of the memory used by Polipo is stored in chunks, fixed-size blocks of memory; the size of a chunk is defined by the compile-time constant CHUNK_SIZE, and defaults to 4096 bytes on 32-bit platforms, 8192 on 64-bit ones.  Chunks are used for storing object data (bodies of instances) and for temporary I/O buffers.  Increasing the chunk size increases performance somewhat, but at the cost of larger granularity of allocation and hence larger memory usage.

By default, Polipo uses a hand-crafted memory allocator based on mmap(2) (VirtualAlloc under Windows) for allocating chunks; while this is very slightly faster than the stock memory allocator, its main benefit is that it limits memory fragmentation. It is possible to disable the chunk allocator, and use malloc(3) for all memory allocation, by defining MALLOC_CHUNKS at compile time; this is probably only useful for debugging.

There is one assumption made about CHUNK_SIZE: CHUNK_SIZE multiplied by the number of bits in an unsigned int (actually in a ChunkBitmap --- see chunk.c) must be a multiple of the page size, which is 4096 on most systems (8192 on Alpha, 65536 on Windows --- go figure).

As all network I/O will be performed in units of one to two chunks, CHUNK_SIZE should be at least equal to your network interface's MTU (typically 1500 bytes).  Additionally, as much I/O will be done at CHUNK_SIZE-aligned addresses, CHUNK_SIZE should ideally be a multiple of the page size.

In summary, 2048, 4096, 8192 and 16384 are good choices for CHUNK_SIZE.

Malloc allocation
=================

Polipo uses the standard malloc(3) memory allocator for allocating small data structures (up to 100 bytes), small strings and atoms (unique strings).

Limiting Polipo's memory usage
==============================

Polipo is designed to work well when given little memory, but will happily scale to larger configurations.  For that reason, you need to inform it of the amount of memory it can use.

Limiting chunk usage
--------------------

You can limit Polipo's usage of chunk memory by setting chunkHighMark and chunkLowMark.

The value chunkHighMark is the absolute maximum number of bytes of allocated chunk memory.  When this value is reached, Polipo will try to purge objects from its in-memory cache; if that fails to free memory, Polipo will start dropping connections.  This value defaults to 24MB or one quarter of the machine's physical memory, whichever is less.

When chunk usage falls back below chunkLowMark, Polipo will stop discarding in-memory objects.  The value chunkCriticalMark, which should be somewhere between chunkLowMark and chunkHighMark, specifies the value above which Polipo will make heroic efforts to free memory, including punching holes in the middle of instances, but without dropping connections.

Unless set explicitly, both chunkLowMark and chunkCriticalMark are computed automatically from chunkHighMark.

Limiting object usage
---------------------

Besides limiting chunk usage, it is possible to limit Polipo's memory usage by bounding the number of objects it keeps in memory at any given time.  This is done with objectHighMark and publicObjectLowMark.

The value objectHighMark is the absolute maximum of objects held in memory (including resources and server addresses).  When the number of in-memory objects that haven't been superseded yet falls below publicObjectLowMark, Polipo will stop writing out objects to disk (superseded objects are discarded as soon as possible).

On 32-bit architectures, every object costs 108 bytes of memory, plus storage for every globally unique header that is not handled specially (hopefully negligible), plus an overhead of one word (4 bytes) for every chunk of data in the object.

You may also want to change objectHashTableSize.  This is the size of the hash table used for holding objects; it should be a power of two and defaults to eight times objectHighMark.  Increasing this value will reduce the number of objects being written out to disk due to hash table collisions.  Every hash table entry costs one word.

OS usage limits
---------------

Many operating systems permit limiting a process' memory usage by setting a usage limit; on most Unix-like systems, this is done with the -v option to the ulimit command. Typically, the effect is to cause calls to the malloc and mmap library functions to fail.

Polipo will usually react gracefully to failures to allocate memory (There are exactly three places in the code where Polipo will give up and exit if out of memory; all three are extremely unlikely to happen in practice.).  Nonetheless, you should avoid using OS limits to limit Polipo's memory usage: when it hits an OS limit, Polipo cannot allocate the memory needed to schedule recovery from the out-of-memory condition, and has no choice other than to drop a connection.

Unfortunately, some operating system kernels (notably certain Linux releases) fail to fail an allocation if no usage limit is given; instead, they either crash when memory is exhausted, or else start killing random processes with no advance warning (How I wish for a SIGXMEM signal.).  On such systems, imposing an (unrealistically large) usage limit on Polipo is the safe thing to do.

Copying
*******

You are allowed to do anything you wish with Polipo as long as you don't deny my right to be recognised as its author and you don't blame me if anything goes wrong.

More formally, Polipo is distributed under the following terms:

     Copyright © 2003--2006 by Juliusz Chroboczek

     Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

     The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

     THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

The last sentence is what happens when you allow lawyers to have it their way with a language.